Ball Joint is a flexible moving joint of the steering assembly. It permits rotating movements in any direction to the part it is installed. It is sealed in a rubber boot filled with grease to ensure a long life span. The front suspension usually has upper and lower ball joints.
The ball joint is a stud with threads on one end and a ball on the other, and the ball seats in a steel socket, with nylon or sintered-iron bearing between the two. It has a flexible ball-and-socket that mounts in the outer ends of the front suspension control arms. It provides a pivoting joint that attaches the steering knuckle to the control arms, which move only up and down. It allows the steering knuckle and wheel to turn in and out for steering. It is sealed by a rubber dust cover that keeps out dust and water and keeps in lubricant. Most ball joints can be lubricated through a grease fitting or removable plug.